ENSC3019 Unit Operations S2 2015

LNG / Gas Process Engineering Example for Process Modules

Acknowledgments: Dr. John Boxall (2012) Various Slides adapted from Terry Edwards, Process Modules lectures Wesfarmers process slides for Process Modelling course Figures GPSA Handbook and Campbell, Gas Conditioning and Process Vol2

Liquefied Natural Gas (LNG) Background

http://energy.gov/sites/prod/files/2013/04/f0/LNG_primerupd.pdf

Liquefied Natural Gas (LNG) Background

http://energy.gov/sites/prod/files/2013/04/f0/LNG_primerupd.pdf

Liquefied Natural Gas (LNG) Background

https://www.spe-qld.org/useruploads/files/aug_2011_final_lng_presentation_rev3_%5Bcompatibility_mode%5D.pdf

Not necessary on NWS

LNG Processing

LNG Storage and Sales

LPG Extraction LPG

Sales

Domestic Pipeline Sales

Ethane Extraction

Petrochemicals & Gas to Liquids

Separation and Processing of Well Fluids

LNG Production – Cascade Refrigeration

Inlet Gas: Methane (and some N2) •  Always requires some front end treatment

–  acid gas removal –  dehydration –  mercury removal

•  Liquid hydrocarbon removal –  Distillation and absorption

•  Liquefaction through a refrigerant cycle

–  Refrigeration –  Heat Exchangers

6 Phillips Optimized Cascade LNG Process

-161 °C

GAS TREATING (SWEETENING)

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Gas sweetening (CO2 removal, also H2S)

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Gas Sweetening Absorption Processes

•  Separate CO2 from NG –  Gas supply at high pressure (>35 bar) –  Typically also removes hydrogen sulphide (H2S) as well

•  Removal specifications –  < 2 % (pipeline) –  < 50 ppm (LNG plant feed)

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Covered in Gas-liquid absorption columns

Gas sweetening: Example

•  CO2/H2S Removal using amine based solvent

•  Information: –  1,000 Sm3/day gas @ 6000 kPa –  0.4% H2S, 3.0% CO2 –  20% solution of DEA

•  What circulation rate is required?

•  Estimate the plant requirements

10 Simplified Design Calcs: GPSA Handbook

Dr, Dc in mm, Q in MSm3/day, P in kPa

Gas sweetening: Example •  QDEA = 360(Qy/x) = 360(1.0*3.4/20) = 61.2 m3/h

–  Add both acid gas concentrations –  1 MSm3 = 1000 Sm3

•  Dc = 10750*sqrt(1.0/√6000) = 1221 = 1200 mm (1.2 m)

–  Based on gas flowrate and density (pressure) –  Quite reasonable diameter for a column

•  Dr = 160*sqrt(61.2) = 1251 = 1300 mm (1.3 m) –  Based on amine flowrate –  Above feed point regen diameter ~67% or 900 mm

•  Reboiler –  H = 93*61.2 = 5690 kW –  A = 4.63*61.2 = 286 m2

–  etc. for other process equipment

Simulation tools providing greater design flexibility/accuracy – Example VMGSim

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DEHYDRATION

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Dehydration to get Natural Gas & Condensate to Shore: 1.  To prevent hydrate formation which would block subsea pipeline

2.  To prevent internal corrosion of subsea pipeline

Dew-point Control or

Glycol Dehydration

Glycol and Molecular Sieve

Dehydration

Dehydration onshore: 1.  As above – but may need even tighter dew-point control

2.  Meet a sales gas water content spec.

Export Gas Pipeline

Dehydration of a gas stream, TEG absorption and regeneration

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What is the saturated water content for a sweet gas that is at 50 °C and 15000 kPa if the gas has a molecular weight of 45 g/mol?

If the first gas processing stage will decrease the temperature to 10 °C without significantly changing the pressure, is dehydration critical prior to this initial processing stage, and if so why?.

Gas Dehydration: Water Content

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Information: Temp 50 °C Pressure 15000 kPa Molecular weight of 45 g/mol

Gas Dehydration

Uncorrected Water Content: 1050 mg water/sm3 wet gas

Corrected Water Content: = 1050 * 0.93 = 977 mg water/sm3 wet gas

Correction for molecular weight of gas / gas density: 0.93

50 °C 0.93

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If the first gas processing stage will decrease the temperature to 10 °C without significantly changing the pressure, is dehydration critical prior to this initial processing stage, and if so why?.

Gas Dehydration

19 (Petrobras Hydrate Plug Removed from Pig-Catcher)

TEG Dehydration example

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A Natural gas stream saturated with water enters a triethylene glycol (TEG) contactor at 50 °C. The gas leaving the contactor must have a water dew point below 0°C. What is the minimum concentration of TEG solution coming back from the regeneration required to dehydrate the gas if a 10 °C approach to equilibrium in the column is assumed? Equilibrium data for TEG contactors is provided in the Figure.

TEG Dehydration example

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Information: Temperature 50 °C Leaving water dew point 0°C 10 °C approach to equilibrium Temperature

~ 99.4 wt% TEG

Physical properties of glycols

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Hysys Example of a TEG Stripper and Regeneration – Process Modelling

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Why are Glycols not enough for LNG?

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Other Dehydration

•  Gas adsorption –  Molecule Sieves common in gas processing –  As pointed out adsorption topics not considered in process

modules

•  Gas Permeation –  Membrane dehydration

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Packed in a tower

Wet Feed Gas in at top

Dry Feed Gas out at bottom

Between 5000kg & 10000 kg in each tower

Mole Sieve gas dehydration

Adsorption and Regeneration Cycling

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Dehydration at Apache’s plant at Varanus Island, NWS

Wet gas

Dry gas

GAS PROCESSING

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Successive distillation/absorption to remove heavier components – mostly just methane for LNG

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Absorption De-ethanizer De-propanizer De-butanizer

To LNG

Westfarmers Straddle NGL plant: Distillation fractions heavier components to LPGs

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Simulation (Hysys etc.) would typically be used for multi-component distillation (MCD) But, McCabe-Thiele Approach for MCD can give initial estimate.

•  Example: de-propanizer

–  0.01 ethane, 0.64 propane, 0.3 butanes, 0.05 pentanes –  top product with < 0.01 butane, bottom <0.02 propane

•  Light Key – Propane

•  Heavy Key – n-Butane

•  Relative Volatility, α = 3.0

•  Non key components: ethane, n-pentane

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Equilibrium line - relative volatility of key components

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Propane Liquid Composition

Propane Vapour Composition

Also need: Operating lines, Reflux Ratio, Feed

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Example Feed: Saturated Vapour – leaving an expansion and heat removed through heat exchanger Assume 65% propane

Lecture 2

Example Reflux Ratio: 1.3

Goal: top product with < 0.01 butane, bottom <0.02 propane

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Rectifying section operating line using reflux ratio

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Y

X

intercept 0.99/(1.3+1) = 0.43

0.99,0.99

(< 0.01 butane)

Feed line for the saturated vapour

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Saturated Vapour – Horizontal Line

(Feed = 65% propane)

0.65 feed (0.64 + 0.01)

Operating line for stripping section

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Bottom composition: 0.02, 0.02

Feed and operating line intersection

(< 0.02 propane)

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McCabe-Thiele MCD w/ binary of Key components

Y

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Draw in Stages

McCabe-Thiele MCD w/ binary of Key components •  Theoretical stages - 29

–  In reality would require more stages –  Efficiency of each stage not 100% - not at equilibrium –  And of course this is only an estimate

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Example Hysys simulation for the production of natural gas liquids (NGL)

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REFRIGERATION

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LNG Production: Refrigeration required to −160 °C

-161 °C

Even NGL significant cooling required LNG - MCHE

Wesfarmers example: Turbo-expander

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Example using the information provided: Expansion from 5900 kPag to 2800 kPag. Inlet Temperature -45 °C, Exit -75 °C (~230, 200 K). Draw Process on P-h diagram (methane) and Estimate Isentropic Efficiency

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out in isentropic isentropich h h ε= +Δ

Expansion in a turbo expander

Need: hout hin hisentropic

Δhisentropic

650 750

605 645 670

605

645

670

-65

( ) /isentropic out in isentropich h hε = − Δ

40 %

Turbo-expansion process

40% Isentropic efficiency reasonable?

•  Short answer, no –  Typically greater than 80%

•  Why?

•  Methane used as the refrigerant for the P-h diagram, example from Wesfarmers is on their NGL train and would contain other components

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Interested in process design? http://www.arrowenergy.com.au/__data/assets/pdf_file/0003/1938/0620-20Project20Description20-20LNG20Plant.pdf

Interested in learning more? http://www.arrowenergy.com.au/__data/assets/pdf_file/0003/1938/0620-20Project20Description20-20LNG20Plant.pdf